9
Delayed nutrient application affects mineralisation rate during composting of plant residues Dorte Bodin Dresbøll a,b, * , Kristian Thorup-Kristensen a a Department of Horticulture, Danish Institute of Agricultural Sciences, Kirstinebjergvej 10, DK-5792 Aarslev, Denmark b Plant Nutrition and Soil Fertility Laboratory, Department of Agricultural Sciences, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark Received 27 February 2004; received in revised form 12 October 2004; accepted 22 October 2004 Available online 8 December 2004 Abstract The hypothesis that delayed addition of nutrient rich material to compost would influence the mineralisation pattern was inves- tigated by studying N turnover in compost based on wheat straw and clover-grass hay. After 7 1 2 weeks of composting almost twice as much N was mineralised when the addition of some of the N-rich clover-grass hay was postponed, suggesting that this influenced the microbial succession. The delayed addition resulted in a second temperature peak and a decline in the pH. Despite the altered con- ditions no significant effect was observed on the weight loss or loss of C and N. In conclusion, compost processes can in a simple way be affected by delayed substrate application leading to a higher nutrient availability without altering other parameters significantly. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Composting; Nitrogen mineralisation; Growing medium; Plant residues; Decomposition 1. Introduction Composting experiments have been performed inten- sively during the last decades. Primarily the studies have focused on rural and urban wastes, often with the aim of reducing volume and avoiding nutrient losses (Witter and Lopez-Real, 1988; Martins and Dewes, 1992; Sa ´n- chez-Monedero et al., 2001). More recently, the focus has also been on composting of plant residues to pro- duce growing media (Jensen et al., 2001; Prasad and Maher, 2001; Garcia-Gomez et al., 2002). Major topics in composting research have been process control and characterisation of maturity or stability criteria. Control of a composting process and the properties of the end product can be achieved by at least two different strate- gies. One strategy is to adjust process parameters, such as moisture level, temperature or oxygen content (Beck-Friis et al., 2001; Sma ˚rs et al., 2002). Another is to alter the starting conditions by changing the compo- sition or type of material used so that C/N ratio or fibre composition is changed (Eklind and Kirchmann, 2000a,b; Eiland et al., 2001). A third strategy, which to our knowledge has not yet been subject to experi- ments, is to influence the composting process by altering the time of addition of parts of the material to be com- posted; normally all the material to be composted is in- cluded right from the start. Nitrogen has often been recognised as a limiting fac- tor for microbial growth and activity during the decom- position of plant residues (Recous et al., 1995), especially in materials with a high C/N ratio such as wheat straw. However, experiments on the effect of additional N supply on the decomposition of plant residues showed different results, ranging from positive to negative effects on the decomposition rate (Fog, 1988). Resource quality, microclimatic conditions and decomposer efficiency are major factors regulating 0960-8524/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2004.10.007 * Corresponding author. Fax: +45 6390 4394. E-mail address: [email protected] (D.B. Dresbøll). Bioresource Technology 96 (2005) 1093–1101

Delayed nutrient application affects mineralisation rate during composting of plant residues

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Page 1: Delayed nutrient application affects mineralisation rate during composting of plant residues

Bioresource Technology 96 (2005) 1093–1101

Delayed nutrient application affects mineralisation rate duringcomposting of plant residues

Dorte Bodin Dresbøll a,b,*, Kristian Thorup-Kristensen a

a Department of Horticulture, Danish Institute of Agricultural Sciences, Kirstinebjergvej 10, DK-5792 Aarslev, Denmarkb Plant Nutrition and Soil Fertility Laboratory, Department of Agricultural Sciences, The Royal Veterinary and Agricultural University,

Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark

Received 27 February 2004; received in revised form 12 October 2004; accepted 22 October 2004

Available online 8 December 2004

Abstract

The hypothesis that delayed addition of nutrient rich material to compost would influence the mineralisation pattern was inves-

tigated by studying N turnover in compost based on wheat straw and clover-grass hay. After 7 12weeks of composting almost twice as

much N was mineralised when the addition of some of the N-rich clover-grass hay was postponed, suggesting that this influenced the

microbial succession. The delayed addition resulted in a second temperature peak and a decline in the pH. Despite the altered con-

ditions no significant effect was observed on the weight loss or loss of C and N. In conclusion, compost processes can in a simple way

be affected by delayed substrate application leading to a higher nutrient availability without altering other parameters significantly.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Composting; Nitrogen mineralisation; Growing medium; Plant residues; Decomposition

1. Introduction

Composting experiments have been performed inten-sively during the last decades. Primarily the studies have

focused on rural and urban wastes, often with the aim of

reducing volume and avoiding nutrient losses (Witter

and Lopez-Real, 1988; Martins and Dewes, 1992; San-

chez-Monedero et al., 2001). More recently, the focus

has also been on composting of plant residues to pro-

duce growing media (Jensen et al., 2001; Prasad and

Maher, 2001; Garcia-Gomez et al., 2002). Major topicsin composting research have been process control and

characterisation of maturity or stability criteria. Control

of a composting process and the properties of the end

product can be achieved by at least two different strate-

gies. One strategy is to adjust process parameters, such

as moisture level, temperature or oxygen content

0960-8524/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biortech.2004.10.007

* Corresponding author. Fax: +45 6390 4394.

E-mail address: [email protected] (D.B. Dresbøll).

(Beck-Friis et al., 2001; Smars et al., 2002). Another is

to alter the starting conditions by changing the compo-

sition or type of material used so that C/N ratio or fibrecomposition is changed (Eklind and Kirchmann,

2000a,b; Eiland et al., 2001). A third strategy, which

to our knowledge has not yet been subject to experi-

ments, is to influence the composting process by altering

the time of addition of parts of the material to be com-

posted; normally all the material to be composted is in-

cluded right from the start.

Nitrogen has often been recognised as a limiting fac-tor for microbial growth and activity during the decom-

position of plant residues (Recous et al., 1995),

especially in materials with a high C/N ratio such as

wheat straw. However, experiments on the effect of

additional N supply on the decomposition of plant

residues showed different results, ranging from positive

to negative effects on the decomposition rate (Fog,

1988). Resource quality, microclimatic conditions anddecomposer efficiency are major factors regulating

Page 2: Delayed nutrient application affects mineralisation rate during composting of plant residues

1094 D.B. Dresbøll, K. Thorup-Kristensen / Bioresource Technology 96 (2005) 1093–1101

composition and activity of decomposer communities

and hence the process of decomposition and nutrient re-

lease (Neely et al., 1991; Agren et al., 2001). Thus, the

effect of added N on decomposition may depend on the

plant material as degradation is influenced by nutrient

content and anatomical structure of the material. Param-eters such as N source and the time scale of the decompo-

sition process also influence the effect of added N.

Recous et al. (1995) found that the ratio of N immo-

bilised to C mineralised decreased with time, and sug-

gested that there was a high N demand during the first

stages of decomposition when soluble and easily degrad-

able C compounds were mineralised, while the N de-

mand was lower when the more recalcitrant Ccompounds were decomposed.

Since much C from plant residues such as straw mate-

rials is only slowly available to microorganisms, leading

to low growth efficiency, a limited amount of N may be

required during decomposition, and recycling of N may

then be adequate to meet the N requirements (Bremer

et al., 1991). Microorganisms, especially fungi, have a

considerable capacity to adapt to N deficient conditions.A large amount of N initially could consequently result

in immobilisation. This greater N immobilisation may

depend on (1) synthesis of microbial biomass with a lower

C/N ratio; (2) higher N losses; or (3) reduced N minerali-

sation or re-mineralisation, which may have been related

to reduced microbial activity (Bremer et al., 1991).

When composting material with the purpose of creat-

ing a growing medium it is important to understand themineralisation and immobilisation processes, as nutrient

release is controlling plant growth. Availability of nutri-

ents from organic composts is often limited despite a

high initial nutrient input, and considerable nutrient

losses frequently occur during the composting period,

primarily due to gaseous emissions. As nutrients are a

limited resource in organic production a more efficient

nutrient use is desirable. Many horticultural plants arevery nutrient demanding and compost used as fertiliser

should provide a high nutrient level from the start. It

was hypothesised that such high efficiency composts

Table 1

The setup for experiments I and II including amounts of material added to e

ratios and water contents of the mixtures

Wheat straw (kg) Clover-g

Experiment I

Treatment 1: all material present initially 32 48

Treatment 2: 1/3 clover-grass initially, 2/3

after 3 weeks

32 16

Treatment 3: 1/6 clover-grass initially, 5/6

after 3 weeks

32 8

Experiment II

Treatment 1: all material present initially 35.5 21.5

Treatment 2: 1/4 clover-grass initially,

3/4 after 3 weeks

35.5 5.5

could be prepared by splitting the addition of the nutri-

ent rich material during the composting process. The

first addition at the start of the composting process

should be sufficient to support the turnover of the read-

ily available carbohydrates. The remaining nutrient rich

material should be added later in the process when theturnover of the wheat straw would already be proceed-

ing. Decomposition of the newly added material would

then result in less N immobilisation compared to com-

post produced by a single addition at the beginning of

the process.

The objective of this study was to test this hypothesis,

by comparing turnover and N release in composts pre-

pared in the above mentioned way with composts pre-pared with all the material present initially.

2. Methods

2.1. Experimental design and materials

Compost was made of wheat straw as structural com-ponent and clover-grass hay as a nutrient rich compo-

nent. The wheat straw was air dried after harvest,

whereas the clover-grass hay was shredded into pieces

of <20mm and oven dried after harvest. Total C and

N of the materials were determined, the wheat straw

having a C/N ratio of approximately 100kg/kg and the

clover-grass hay a C/N ratio of 15kg/kg. Initial C/N

ratio of the composts was calculated based on theamount of organic material and the total C and N con-

tent of the materials.

Two experiments were set up as summarised in Table

1. Experiment I had three different treatments with three

replicates in each. Treatment 1 was a mixture of clover-

grass hay and wheat straw giving a C/N ratio of 25.

Treatment 2 had only one third of the clover-grass hay

from the beginning and thus an initial C/N ratio of 38.Treatment 3 had even less, only one sixth of the clover-

grass hay initially, which resulted in a C/N ratio of 50.

After three weeks of composting, when temperature

ach compost box in the different treatments, and the initial C/N mass

rass initial (kg) Clover-grass 3 weeks (kg) C/N Water (%)

0 25 78

32 38 79

40 50 77

0 35 54

16 60 52

Page 3: Delayed nutrient application affects mineralisation rate during composting of plant residues

D.B. Dresbøll, K. Thorup-Kristensen / Bioresource Technology 96 (2005) 1093–1101 1095

had decreased to 20 �C indicating less microbial activity

in treatments 2 and 3, supplemental clover-grass hay

was added in these treatments. Hence, in the end all

three treatments had received the same amount of clo-

ver-grass hay and wheat straw in total.

In experiment II only two different treatments withthree replicates were compared as no significant differ-

ences were found between treatments 2 and 3 at the

end of experiment I. The N content was reduced in this

experiment as compost from experiment I was too nutri-

ent rich as a growing medium. Treatment 1 was a mix-

ture of wheat straw and clover-grass hay giving a C/N

ratio of 35. Treatment 2 contained only a quarter of

the clover-grass hay initially resulting in a C/N ratio of60. Three weeks later the remaining clover-grass hay

was added.

The wheat straw was cut into pieces of <10cm and

the substrates were mixed in a compost mixer and

watered to a water content of approximately 70%.

Leaching losses during mixing were avoided by placing

some of the straw material beneath the mixer to absorb

runoff. After mixing, the straw material was added tothe compost boxes. Additional water was added when-

ever necessary during the composting time to keep the

water contents above 50%. The composting experiments

were performed in 800L wooden boxes measuring

0.7 · 1.0 · 1.2m (height · width · length). Heat loss

was minimised by insulation with glass–wool mats and

the boxes were passively aerated by heat convection.

Total composting time was 7 12weeks in experiment I

and 8 weeks in experiment II. After three weeks, the

compost of both experiments was turned and watered

in the compost mixer and the supplemental clover-grass

was added.

2.2. Sampling

Every 1 12-weeks, samples were taken for analysis of

water content, mineralised N, total N and C content,

and pH in experiment I. In experiment II sampling

was conducted every week. Five samples of approxi-

mately 100g were collected from each box in experiment

I, pooled and mixed. Subsamples were taken from the

pooled sample for each of the analyses. In order to

improve sampling techniques ten smaller samples of

approximately 5g were taken from each compost boxat a depth of 0.2–0.5m and pooled for each analysis in

experiment II. The entire sample was used for the ana-

lysis. In this way, each analysis was conducted on mate-

rial randomly collected throughout the compost matrix

and should provide an average of the compost in the

box. Samples were collected randomly from the boxes

except for the peripheral zone of 0.1m in both experi-

ments. At the beginning, after three weeks and at termi-nation of the experiments all boxes were weighed to

determine the total weight loss. Temperature was mea-

sured continuously in the centre of the composting

boxes using standard acid proof stainless steel Pt-100

probes connected to a data logger (Datataker DT500).

2.3. Physical and chemical analysis

Water content was determined by weight loss of com-

post samples, which were oven dried at 80 �C for 24h.

Total N and C were measured by dry combustion of

dried and finely ground samples with an automated

N–C analyser interfaced with an isotope mass spectro-

meter (Carlo Erba, EA 1108). Samples were acidified

by pouring 60ml 0.1M HCl over the entire sample be-

fore drying to avoid NHþ4 losses.

The actual concentrations of NHþ4 and NO�

3 in the

compost were measured after each sampling. Compost

samples (20g fresh weight) were analysed for NHþ4

and NO�3 content in a 2M KCl extract (compost: solu-

tion ratio 1:10) followed by shaking for 45min and cen-

trifugation. The supernatant was filtered through

Advantec 6 (Frisenette Aps.) filters and stored frozen.

As the extracts had a dark colour due to organic acids,they were cleared by shaking the extract with active C

for 15min followed by filtration (0.45lm pore size) to

avoid interference in spectrophotometric measurements.

The analysis for NHþ4 and NO�

3 content was conducted

by standard colorimetric methods using flow-injection

analysis (FIA) (Keeney and Nelson, 1982). Ammonium

was measured by high pressure liquid chromatography

(HPLC) in experiment I. pH was measured in a solutionof compost (20g fresh weight) and water in a ratio of

1:5.

2.4. Statistics

The results were calculated as an average of three rep-

licates of each treatment. Data were log transformed to

obtain homogeneity of variance and analysed with theGLM procedure of the SAS statistical package (SAS

Institute Inc., Cary, NC, USA).

3. Results

3.1. Process parameters

3.1.1. Temperature

In treatment 1 of experiment I temperature increased

rapidly to thermophilic conditions peaking at 68 �C (Fig.

1a). Temperatures remained above 40 �C for more than

three weeks. Turning the compost did not result in a

temperature increase in this treatment. In treatment 1

of experiment II the initial increase in temperature to

thermophilic conditions only lasted less than a week(Fig. 1b). In treatments 2 and 3 of experiment I the ther-

mophilic phase lasted 8–10 days, after which the temper-

Page 4: Delayed nutrient application affects mineralisation rate during composting of plant residues

Time (weeks)

0 2 4 6 8 10

Tem

pera

ture

(oC

)

0

20

40

60

80

123

Time (weeks)

0 2 4 6 8 10

12

(a) (b)

Fig. 1. Temperature development during composting in (a) experiment I: treatment 1: all material present initially (——), treatment 2: 1/3 clover-

grass hay added initially, 2/3 after 3 weeks (� � �), and treatment 3: 1/6 clover-grass hay added initially, 5/6 after 3 weeks (- - -). (b) shows temperature

development in experiment II: treatment 1: all material present initially (——), and treatment 2: 1/4 clover-grass hay added initially, 3/4 after 3 weeks

(� � �).

1096 D.B. Dresbøll, K. Thorup-Kristensen / Bioresource Technology 96 (2005) 1093–1101

ature continued to decrease. After addition of the sup-plementary clover-grass hay the temperature increased

again to 70 �C followed yet again by a fast decline

(Fig. 1a). The temperature pattern in treatment 2 of

experiment II resembled the pattern of treatments 2

and 3 of experiment I although the temperature maxi-

mum was significantly lower in treatment 2 of experi-

ment II.

3.1.2. Compost pH

pH varied between 7.6 and 8.9 in treatment 1 of

experiment I and the highest values were found after

3–4 weeks (Fig. 2a), when the NHþ4 concentration was

also high. Towards the end of the experiment the pH de-

clined to 8.4. Treatments 2 and 3 followed the same pat-

tern, with the exception that after 3 weeks when the

additional clover-grass hay was added the pH declined

Time (weeks)

0 2 4 6 8

pH

6

7

8

9

10123

(a)

Fig. 2. Changes in pH during composting of wheat straw and clover-grass

treatment 2: 1/3 clover-grass hay added initially, 2/3 after 3 weeks (s), and tre

(b) experiment II: treatment 1: all material present initially (d), and treatmen

are the mean of three replicates and bars show the standard deviation.

to 6.6–6.7. In treatment 1 of experiment II pH varied be-tween 7.3 and 8.9, whereas pH in treatment 2 varied be-

tween 7.5 and 8.5. Towards the end of the composting

period a small decrease was observed in treatment 1

(Fig. 2b).

3.1.3. Weight loss

After three weeks of composting, before additional

clover-grass was added, no significant differences werefound in weight losses between the treatments in exper-

iment I (44–45% of initial weight). In experiment II

weight losses after three weeks were only half the losses

in experiment I and as in experiment I no significant dif-

ference between the treatments was observed (Table 2).

After 7 12weeks weight losses in all treatments were

about 61–63% of initial weight in experiment I, whereas

the weight losses in experiment II after 8 weeks were

Time (weeks)

0 2 4 6 8

12

(b)

hay in (a) experiment I: treatment 1: all material present initially (d),

atment 3: 1/6 clover-grass hay added initially, 5/6 after 3 weeks (.) and

t 2: 1/4 clover-grass hay added initially, 3/4 after 3 weeks (s). Results

Page 5: Delayed nutrient application affects mineralisation rate during composting of plant residues

Table 2

Weight and carbon losses after three weeks of composting and at the end of the composting experiments, and N losses at the end of the experiments

3 Weeks Final

Weight loss (%) C loss (%) Weight loss (%) C loss (%) N loss (%)

Experiment I

Treatment 1: all material present initially 46 (6) 50 (7) 61 (6) 60 (6) 4 (16)

Treatment 2: 1/3 clover-grass initially, 2/3 after 3 weeks 45 (5) 52 (6) 63 (5) 63 (6) 12 (10)

Treatment 3: 1/6 clover-grass initially, 5/6 after 3 weeks 44 (7) 52 (8) 61 (1) 62 (2) 9 (3)

Experiment II

Treatment 1: all material present initially 16 (5) 24 (11) 48 (9) 54 (3) 30 (24)

Treatment 2: 1/4 clover-grass initially, 3/4 after 3 weeks 26 (8) 32 (3) 54 (5) 58 (5) 22 (11)

Results are means of three replicates. Standard deviations are shown in parentheses.

Time (weeks)0 2 4

C/N

rat

io

0

20

40

60

12

C/N

rat

io

0

20

40

60

80

123

(a)

(b)

6 8

Fig. 3. Changes in C/N ratio during composting of wheat straw and

clover-grass hay in (a) experiment I: treatment 1: all material present

initially (d), treatment 2: 1/3 clover-grass hay added initially, 2/3 after

3 weeks (s), and treatment 3: 1/6 clover-grass hay added initially, 5/6

after 3 weeks (.) and (b) experiment II: treatment 1: all material

present initially (d), and treatment 2: 1/4 clover-grass hay added

initially, 3/4 after 3 weeks (s). Results are the mean of three replicates

and bars show the standard deviation.

D.B. Dresbøll, K. Thorup-Kristensen / Bioresource Technology 96 (2005) 1093–1101 1097

48% in treatment 1 and 54% in treatment 2; however, the

difference was not statistically significant (Table 2).

3.2. Carbon and nitrogen loss

Total loss of C was determined from the changes in

compost total C content over the experimental periods.

After three weeks of composting no significant differ-ences were observed between the treatments in either

of the experiments. After 7 12weeks the C losses did

not vary significantly between the three treatments in

experiment I whereas the difference between the two

treatments in experiment II after 8 weeks was more pro-

nounced although not statistically different (Table 2). As

observed in the weight losses, the C losses were signifi-

cantly higher in experiment I compared to experimentII.

Nitrogen losses were determined as the mass balance

of N and reflected both N gaseous emissions and N

leaching. As the water content was regulated carefully

throughout the composting period the losses due to

leaching were minimal. Nitrogen losses in experiment I

were low with less than 13% of total N lost during the

composting period. In experiment II higher N losseswere observed in both treatments with up to 30% losses

(Table 2). No significant differences were observed

between the treatments in either of the experiments.

The percentage of C in the compost of both experiments

was quite constant at around 46% during the compo-

sting period, although there was a slight tendency to a

reduction in the C concentration (results not shown).

The percentage of N increased from 2.8% to 4.6% intreatment 1 of experiment I whereas it increased from

1.7–1.8% to 4.6% in the other treatments. In experiment

II the increase was less steep from 1.3% to 2.2% in treat-

ment 1 and from 0.8% to 2.5% in treatment 2. These re-

sults are reflected in the C/N ratios (Fig. 3).

3.3. Nitrogen mineralisation

Postponing the addition of some of the nutrient rich

material had a significant effect on the timing and extent

of the mineralisation processes in experiment I. When all

the clover-grass hay was added initially, the NHþ4 con-

centration increased during the first weeks, and declined

afterwards (Fig. 4b). The NO�3 concentration remained

low for the first three weeks, followed by a steady

Page 6: Delayed nutrient application affects mineralisation rate during composting of plant residues

Tot

al in

orga

nic

N (

µg g

-1 D

W)

0

1000

2000

3000

4000

NH

4+-N

(µg

g-1

DW

)

0

1000

2000

3000

4000

123

Time (weeks)1 2 3 4 5 6 7 8

NO

3--N

(µg

g-1

DW

)

0

1000

2000

3000

4000

(a)

(b)

(c)

Fig. 4. Mineralisation pattern during composting of wheat straw and

clover-grass hay in experiment I: (a) total mineralised nitrogen, (b)

ammonium content, and (c) nitrate content. Treatment 1: all material

present initially (d), treatment 2: 1/3 clover-grass hay added initially,

2/3 after 3 weeks (s), and treatment 3: 1/6 clover-grass hay added

initially, 5/6 after 3 weeks (.). Results are means of three replicates

and bars show the standard deviation.

Tota

l ino

rgan

ic N

(µg

g-1 DW

)

0

50

100

150

200

Time (weeks)

0 2 4 6 8 10

NO

3- -N (µ

g g-1

DW

)

0

50

100

150

200

NH

4+ -N

(µg

g-1 D

W)

0

50

100

150

200

12

(a)

(b)

(c)

Fig. 5. Mineralisation pattern during composting of wheat straw and

clover-grass hay in experiment II: (a) total mineralised nitrogen, (b)

ammonium content, and (c) nitrate content. Treatment 1: all material

present initially (d), and treatment 2: 1/4 clover-grass hay added

initially, 3/4 after 3 weeks (s). Results are means of three replicates

and bars show the standard deviation.

1098 D.B. Dresbøll, K. Thorup-Kristensen / Bioresource Technology 96 (2005) 1093–1101

increase (Fig. 4c). Postponing the addition of some of

the clover-grass hay resulted in an altered mineralisation

pattern. The NHþ4 concentration increased slightly dur-

ing the first three weeks followed by an increase when

the rest of the clover-grass hay was added (Fig. 4b).

After five to six weeks a decline in NHþ4 was observed.

At the same time a steep increase in NO�3 concentration

was seen after six weeks of no NO�3 production (Fig. 4c).

At the end of the experiment the total mineralised N

concentration was significantly higher (p < 0.05) after

the postponed addition (Fig. 4a). After 7 12weeks the

inorganic nitrogen content in the compost in treatments

2 and 3 (Fig. 4) corresponded to about 3.5% of the ini-

tial total N being mineralised, whereas only about 1.6%

of initial total N was mineralised in treatment 1. In

experiment II only a small amount of NHþ4 was detected

during the composting period, whereas practically no

NO�3 was found (Fig. 5b and c), hence after 8 weeks

the inorganic content corresponded to 0.15% of the ini-

tial amount of total N being mineralised. After 7 12weeks

of composting in experiment I, the NHþ4 =NO�

3 ratios

were 0.24, 0.32 and 0.25 for treatments 1, 2 and 3 respec-

Page 7: Delayed nutrient application affects mineralisation rate during composting of plant residues

D.B. Dresbøll, K. Thorup-Kristensen / Bioresource Technology 96 (2005) 1093–1101 1099

tively. In contrast, after 8 weeks of composting the

NHþ4 =NO�

3 ratios in experiment II were much higher,

2.4 in treatment 1 and 3.6 in treatment 2.

3.4. Sampling

Variation between replicates was high, and altering

the sampling procedure did not minimise the variation.

This could probably be explained by uncertain determi-

nation of the dry matter content of the total amount of

compost in the boxes, influencing most of the measured

parameters.

4. Discussion

The mineralisation in treatment 1 in experiment I fol-

lowed a pattern often observed during decomposition in

compost (Eklind and Kirchmann, 2000b). During the

microbially very active initial phase NHþ4 can be accu-

mulated, which can result in an elevation of the pH, as

mineralisation of organic N is a proton assimilating pro-cess (Beck-Friis et al., 2003). This was observed in both

experiments. The combination of high pH, high NHþ4

concentrations and high temperatures promote NH3

volatilisation and the highest ammonia losses occur dur-

ing this phase (Witter and Lopez-Real, 1987; Martins

and Dewes, 1992; Beck-Friis et al., 2003). The following

decrease in pH coincided with NO�3 production as the

nitrification process releases protons.Nitrate contents were not measurable until after the

initial three weeks of composting as high temperatures

inhibit the nitrifying bacteria (Willers et al., 1998).

Nitrifying bacteria probably survived in the peripheral

zone of the composting boxes where the temperature

was not as high as in the centre of the compost pile.

Nitrate content in the compost increased after the tem-

perature decreased and the compost was turned, sug-gesting that nitrifying bacteria were mixed into the

compost.

Despite the presence of conditions permitting ammo-

nia losses the total N losses were low, 4–29% of initial N

content. Compared to what is normally seen during

composting these losses were relatively small. During

composting of animal manure, household waste, and

other waste products, losses of at least 50% may beobserved (Witter and Lopez-Real, 1988; Martins and

Dewes, 1992; Eklind and Kirchmann, 2000b). The low

N losses in these experiments could partly be explained

by the experimental set-up as the compost was not

rotated and aerated as much as compost in reactors

(Eklind and Kirchmann, 2000a; Beck-Friis et al., 2003)

or in heaps (Martins and Dewes, 1992; Sommer,

2001). More importantly, the materials used had low ini-tial NHþ

4 contents compared to many waste materials

with high NHþ4 contents or easily degradable com-

pounds such as urea and thus a higher risk of losses

when temperature and pH increase (Noble et al., 2002).

Postponing the addition of some of the nutrient rich

material altered the mineralisation patterns significantly

in experiment I. During the first three weeks the NHþ4

content was low as no net mineralisation occurred,probably because inorganic N was immobilised during

the degradation of soluble and easily degradable carbo-

hydrates. Recous et al. (1995) observed a decrease in

the ratio of N immobilised to C mineralised with time,

confirming the high initial N demand. When the sup-

plemental clover-grass hay was added after three weeks

an increase in NHþ4 content was observed, indicating

that the organic N from the clover-grass hay amend-ment was mineralised and N immobilisation was lower

than in treatment 1. These results support the hypo-

thesis that a limited amount of N is needed initially

in the decomposition of the readily available carbohy-

drates of the straw material (Bremer et al., 1991). Usu-

ally, bacteria degrade the soluble compounds during

the initial phases of decomposition, whereas fungi with

a higher C/N ratio decompose more recalcitrant com-pounds (Recous et al., 1995; Klamer and Baath,

1998). The fungal/bacteria index increases during the

decomposition of material with a high initial C/N ratio.

The same phenomenon occurs during the initial phases

of composting (Eiland et al., 2001), confirming the fun-

gal dominance in degrading recalcitrant compounds.

When the additional N was added the readily available

carbohydrates were presumably already degraded andless N demanding fungi dominated the decomposition.

Thus, when the N was mineralised from the supplemen-

tal clover-grass hay it was not re-immobilised by the

microbial population to the same degree as when all

clover-grass hay was added initially. Therefore the de-

layed addition of clover-grass hay resulted in a higher

total release of inorganic N during the experimental

period. The NO�3 content, however, remained low for

three weeks more than in treatment 1.

The initial N level was reduced in experiment II,

resulting in a N level which was probably so low that al-

most no net mineralisation occurred. Only a small net

production of NHþ4 occurred during the first three weeks

in treatment 1, after which immobilisation was detected

(Fig. 5a and b). During the first three weeks, treatment 1

of experiment II was comparable to treatment 2 ofexperiment I, having similar initial C/N ratios. Hence,

presumably sufficient N was available for the initial bac-

terial decomposition of soluble compounds in treatment

1 of experiment II. The increase in temperature as well

as the C and weight losses indicated considerable micro-

bial activity. During decomposition of plant material

in soil, Recous et al. (1995) observed that if mineral N

was not available the C decomposition decreased butit did not stop. Despite the low N content in treatment

1 of experiment II the C mineralisation proceeded

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1100 D.B. Dresbøll, K. Thorup-Kristensen / Bioresource Technology 96 (2005) 1093–1101

throughout the experimental period, suggesting that the

initial low N content could have altered the microbial

succession. Decomposition might have been dominated

by fungi which may have the ability of effective remobil-

isation and transfer of N to actively growing parts

(Cowling and Merrill, 1966). As no supplemental clo-ver-grass hay was added in treatment 1 of experiment

II no considerable net N mineralisation was observed

after the initial three weeks.

Total C losses resembled losses normally seen during

decomposition of plant material (Bremer et al., 1991). It

could have been expected that the mass and C losses

would be lower in treatments 2 and 3 than in treatment

1 in experiment I, with the delayed addition of nutrientrich material, since the decomposition was expected to

proceed more slowly but steadily after the initial decom-

position of the readily available carbohydrates. How-

ever, no decrease of the mass and C loss was seen in

treatments 2 and 3 compared to treatment 1. This could

be explained by the higher microbial activity in treat-

ments 2 and 3 when the supplementary clover-grass

hay was added, indicated by the temperature increase.In addition, a small increase in temperature normally

occurs when the compost is turned due to better aera-

tion and humidity conditions or simply due to the whirl.

During composting the C/N ratio decreased to

around 10 in all three treatments of experiment I, which

indicated the biological stability of the composts (Bernal

et al., 1996). The C/N ratio can be used as a compost

maturity parameter implying a stable organic mattercontent and absence of phytotoxic compounds (Bernal

et al., 1998). Another maturity parameter is the ratio

between NHþ4 -N and NO�

3 -N as decreasing amounts

of NHþ4 -N combined with increases in NO�

3 -N concen-

trations towards the end of composting suggest that

intensive biological decomposition has been completed

(Pare et al., 1998). This shift in inorganic N was seen

in experiment I, although the ratio was just above thesuggested maturing index of 0.16 (Bernal et al., 1996).

In experiment II on the other hand, the C/N ratio never

declined to less than 20 and no NO�3 was produced, con-

firming that decomposition became N limited. Addition-

ally, the high NHþ4 =NO�

3 ratio is indicative of immature

compost.

5. Conclusions

In conclusion, postponing the addition of nutrient

rich material affected the mineralisation pattern result-

ing in more plant available N after 7 12weeks of compo-

sting. Total losses of mass, C and N were not affected

significantly by the delayed addition. This suggests that

without altering the amount or type of material to becomposted, mineralisation can be managed by simple

methods, which could be of great importance when

growing plants with a high initial N demand. There

seemed, however, to be a critical N addition below

which net mineralisation was not obtained.

Acknowledgement

We thank Stig Sandholt Andersen, Jens Barfod and

Birthe Flyger for skilful technical assistance as well as

Hanne Lakkenborg Kristensen and Jakob Magid for

valuable comments on the manuscript. Financial sup-

port was provided by the Danish Research Centre for

Organic Farming (DARCOF).

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